Although catalysts for the conversion of hydrocarbons have a tendency to deactivate, usually a catalyst's activity may be restored by one of a number of processes that are known generally as regeneration processes. Regeneration processes are extensively used. What specific steps comprise a regeneration process depend in part on the reason for the deactivation. For example, if the catalyst deactivated because coke deposits accumulated on the catalyst, regeneration usually includes removing the coke by burning. If the catalyst deactivated because a catalytic metal such as platinum became agglomerated, regeneration usually includes redispersing the metal by contacting the catalyst with oxygen and chlorine. If the catalyst deactivated because a catalytic promoter such as chloride became depleted, regeneration usually includes replenishing the promoter by contacting the catalyst with a chlorine-containing species. Operating conditions and methods for these regeneration processes are well known. Regeneration processes can be carried out in situ, or the catalyst may be withdrawn from the vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for reactivation. Arrangements for continuously or semicontinuously withdrawing catalyst particles from a reaction zone and for reactivation in a regeneration zone are well known.
Many of these regeneration processes share the common feature of introducing one or more chlorine-containing compounds into the regeneration zone in order to restore the activity of the catalyst for use in the reaction zone. Although chlorine is sometimes introduced into the regeneration zone, it is much more common that one of several chlorine-containing compounds, such as 1,1 dichloroethane, 1,2 dichloroethane, 1,1 dichloropropane, and 1,2 dichloropropane, is introduced into the regeneration zone. The most commonly used compounds thus contain not only chlorine but also carbon and/or hydrogen. Many regeneration zones into which these compounds are introduced typically contain molecular oxygen and operate at conditions that have been carefully optimized with a view towards combusting coke deposits on the catalyst or towards oxidizing or dispersing a catalytic metal on the catalyst. When a chlorine-containing compound is introduced into such a regeneration zone, it also is generally combusted or oxidized, and by-products of combustion, such as carbon dioxide, water, hydrogen chloride, and chlorine, are formed. As the chlorine-containing compound combusts, regions of intense burning can arise in the regeneration zone, either in portions of the catalyst and/or near to mechanical internals within the regeneration zone.
Two problems associated with localized regions of intense combustion of the chlorine-containing compound within the regeneration zone are catalyst deactivation and mechanical failure. As to catalyst deactivation, the combination of temperature, water vapor, and exposure time determine the useful life of the catalyst. Exposure of high surface area catalyst to high temperatures for prolonged periods of time will transform the catalyst into a more amorphous material that has a decreased surface area. Decreased surface area in turn can lower the activity of the catalyst to a level at which the catalyst is considered deactivated. This type of catalyst deactivation is permanent and can eventually render the catalyst unusable. Similarly, with respect to mechanical failure, the exposure of the internal mechanical parts of the regeneration zone to high temperatures for extended periods of time will change the physical properties of the parts and degrade or weaken their structural integrity. Consequently, the internal parts can break or crack, thereby necessitating costly repairs and downtime.